Summoning and Polymorph - two completely new, fan-voted skill trees - are now in the Early Access build of Divinity: Original Sin 2.
I'm still trying to get my head around them. Summoning summons totems or incarnates to help you in battle, whereas Polymorphing physically alters you, granting new abilities.
Totems are a bit like turrets that fire an appropriate attack each round - ie. oil totems shoot an oil attack to slow enemies and fire totems shoot fireballs.
Incarnates, on the other hand, are pets. They have a baseline ability but can be buffed by infusing them with power, which gives them extra abilities. A water incarnate will heal you as a baseline ability, for example, but can also attack if you infuse it with power.
The Polymorph skill tree, meanwhile, grants things like horns, allowing you to charge; wings, allowing you to fly over surfaces and also great distances; and a tentacle arm to disarm people. There's steel skin, chameleon invisibility and, best of all, you can turn enemies into chickens.
There's a short reveal video as well as more in depth explanation video by Swen Vincke, head of Larian, who I can tell you with authority walks around the office in socks.
Divinity: Original Sin 2 is due this year on PC, though there's no concrete date. Consoles may follow, as they did with Original Sin 1. I wrote a Divinity: Original Sin 2 preview a few months ago. Suffice to say I'm a big fan.